1. Field of the Invention
The present invention relates to fuel pumps for supplying fuel to internal combustion engines. More particularly, the present invention relates to a fuel pump having a plunger bore and seal configured to facilitate an increase in the pressurized region along the length of the pump plunger bore.
2. Description of the Related Art
Today's engine designers must meet the challenge of government mandated emissions criteria while striving to improve engine fuel efficiency. In rising to this challenge, designers create fuel systems that operate at higher pressures than systems of the past. In so doing, greater performance demands are placed on fuel pump components and operations. Fuel pumps typically include a pump plunger positioned in the bore of a fuel pump barrel and sized so as to permit reciprocating motion within the bore. Pump plungers are driven by a drive system positioned in a separate mechanical compartment supplied with lubricating oil. Because the plunger diameter must necessarily be less than the bore diameter, fuel leakage in the resulting space can occur. Fuel escapes from the fuel pumping chamber and passes along the clearance space between plunger and bore, then is available to leak into the drive compartment. Such fuel leakage contaminates the engine lube oil and causes a reduction in the oil's viscosity, thus shortening its life and effectiveness.
To address the problem of fuel leakage, traditional engine designs provide a drain groove located in the plunger bore between the pumping chamber and the end of the bore opposite the pumping chamber. The portion of the bore between the pumping chamber and drain groove provides a high-pressure seal by virtue of the close tolerance between plunger and bore diameters. Fuel leaking through the clearance area above the drain groove is collected by the drain groove and diverted to a fuel drain circuit. The portion of the bore between the drain groove and the bore end opposite the pumping chamber is formed as an annular clearance gap between the barrel and plunger, and serves to separate the drain groove from the lube oil. To prevent fuel flowing out of the drain groove along the bore in the clearance between the barrel and the plunger from reaching the lube oil, some traditional pump designs provide a back-up seal. The back-up seal also serves to inhibit lube oil from entering the fuel chamber. However, this approach has the disadvantage of pressure spikes occurring due to dilation of the plunger under axial load. Such pressure spikes hinder the operation of the seal. Accordingly, what is needed is a fuel pump that can provide adequate pressurization to meet modern design standards yet employ a sealing system that satisfactorily preserves the integrity of both the fuel and lube oil areas.